Types of Eye Movement

1
Describe 2 functions of eye movements and give an
example of each.
Types of Eye Movement
Information Gathering Stabilizing Voluntary
(attention) Reflexive Saccades vestibular
ocular reflex (vor) new location, high velocity,
ballistic body movements Smooth
pursuit optokinetic nystagmus (okn) object
moves, velocity, slow whole field image
motion Vergence change point of fixation in
depth slow, disjunctive (eyes rotate in opposite
directions) (all others are conjunctive)
Fixation period when eye is relatively
stationary between saccades.
2
Describe foveal over-representation in the visual
cortex. Give a reason for this
over-representation. Draw a diagram to
illustrate your answer.
3
Describe foveal over-representation in the visual
cortex. Give a reason for this
over-representation. Draw a diagram to
illustrate your answer.
Cortical Magnification
Primary Visual Cortex V1
Larger cortical area for neural projections from
the fovea than periphery
4
Give a reason for this over-representation.
- High density of cone photoreceptors in
central fovea.
5
Describe your results in the ball-catching lab.
6
Draw a sketch of the brain showing the structures
involved in the generation of a saccadic eye
movement. Specify the function of these
structures (to the extent that this is possible)
7
Function of Different Areas
monitor/plan movements
target selection
saccade decision
saccade command
inhibits SC
signals to muscles
8
Sketch a muscle, including the muscle spindle and
the sensory and motor nerve fibers.
9
MUSCLEFIBRE/SPINAL
Secondary Encoding (Less Sensitive)

• Spindles

Golgi
Dorsal
Extrafusal
Ventral
Intrafusal
10
How does the speed of a movement (e.g. reaching)
affect its accuracy? Explain why.
Role of Visual Feedback
Question why does error increase with
speed? Note 50 cm/sec 5cm/100msec
11
Draw a sketch of a feedback system and a
feedforward system. Give an example of each (for
visuo-motor control).
12
Schematic Representation of Feedback and
Feed-forward Systems
Eg pursuit, reaching, grasping
Eye velocityimage velocity
Motor command
sensory
retinal velocity
delay
Eg saccade, throwing
Load/fatigue/current position
wind
ballistic
Learnt motor command
guided
13
Describe the effect on reaching movements of
large-fiber sensory neuropathy (degeneration of
the afferent fibers from the muscles and skin)
when visual feedback is eliminated? Sketch the
movements to illustrate your answer.
14
Consequences of loss of feedback on reaching
Large fibre sensory neuropathy leads to loss of
proprioceptive feedback from muscles
Errors in direction,distance
Normal proprioception only
No vision or proprioception
Vision compensates for lack of proprioception
15
Describe the problems that have to be solved in
order to program a robot to flip an egg. Mention
possible solutions.
16
Flipping an Egg
Autonomous control robot is pre-programmed - no
human input
Problems to be solved
1. Grasp spatula
locate handle (vision) some mechanism to
translate location into arm movement some
mechanism for controlling fingers - passive
compliance
2. Move to pan
locate pan (vision) translate location into arm
movement
3. Lower spatula to pan
vision or proprioception lower until force gt 0
4. Flatten
proprioception rotate until forces on fingers
are equal
5. Locate egg
vision or proprioception move forwards until
horizontal force gt 0
17
Flipping an Egg (ctd)
Problems to be solved
6. Lift
need to keep spatula level vision or
proprioception (keep tension constant)
7. Flip
need to learn how much to rotate hand rotate
until forces 0
Limitations of autonomous control
inflexible - cant adapt to changed
circumstances requires high precision
18
Why is prediction necessary?
Components of visuo-motor latency.
Photoreceptors ganglion cells
LGN primary visual cortex posterior
parietal ctx pre-motor ctx M1
muscles
Round trip from eye to brain to muscles takes a
minumum of 200 msec. Ball (our expt) only takes
about 900 msec. Prediction gets around the
problem of sensory delays.
19
Ability to adapt to new relationships requires
cerebellum
20
Why do we need to retain plasticity for new
visuo-motor relationships?
1. Need to adjust to changes in body size during
development.
2. Need to adjust to damage/aging.
3. Need to adjust to environmental changes eg
ice, loads etc.
4. Need to learn arbitrary mappings for tool use
etc.
5. Need to acquire new motor skills.
6. Visuo-motor coordination is a computationally
difficult problem for the brain. Need flexibility
to correct errors.
21
Role of Experience in Development of Visuo-motor
coordination
Held Hein
1
2

• Both kittens get visual experience and motor
  experience
• Visual experience correlated with motor
  commands/proprioceptive feedback/vision of limbs
• Gets both, but uncorrelated. Kitten 2 -abnormal
  visuo-motor coordination.

22
Adaptation to different relation between vision
and movement.

• George Stratton
• Wore inverting lens for 8 days

If he saw an object on the right he would reach
with his right hand and discover he should have
reached with his left. He could not feed himself
very well, could not tie his shoelaces, and found
himself severely disoriented. His image of his
own body became severely distorted. At times he
felt his head had sunk down between his
shoulders,and when he moved his eyes and head the
world slid dizzyingly around.
As time went by Stratton achieved more effective
control of his body. If he saw an object on the
right he would reach with his left hand. He could
accomplish normal tasks like eating and dressing
himself. His body image became almost normal and
when he moved his eyes and head the world did not
move around so much. He began to feel as though
his left hand was on the right, and his right
hand on the left. If this new location of his
body was vivid, the world appeared right side up,
but sometimes he felt his body was upside down in
a visually right-side-up world. After removing
the prisms, he initially made incorrect reaching
movements. However, he soon regained normal
control of his body.
23
Adaptation to different relation between vision
and movement.

• George Stratton
• Wore inverting lens for 8 days
• Believed that we learn visual directions by
  associating visual experiences with other forms
  of sensory feedback (e.g. proprioceptive).
• Alternatively
• Adaptation results from learning correlation
  betweeen vision and actively generated motor
  commands (Held, 1965).

24
Role of Experience in Development of Vision
Molyneuxs Question Can a person blind from
birth, whose vision is restored, tell that a
circle and a square are different shapes?
25
Mike May - world speed record for downhill skiing
by a blind person. Lost vision at age 3 -
scarred corneas. Optically 20/20 - functionally
20/500 (cf amblyopia) Answer to Molyneuxs
question Mike May couldnt tell difference
between sphere and cube. Improved, but does it
logically rather than perceptually. (cf other
cases) Color an orange thing on a basket ball
court must be a ball. Motion can detect moving
objects, distinguish different speeds. Note
fMRI shows no activity in Infero-temporal cortex
(corresponding to pattern recognition) but there
is activity in MT, MST (motion areas) and V4
(color). Other parts of brain take over when a
cortical area is inactive. Cannot recognize
faces. (eyes, movement of mouth
distracting) Cant perceive distance very
well. Cant recognize perspective. No size
constancy or lightness constancy/ segmentation of
scene into objects, shadows difficult. Vision
most useful for catching balls (inconsistent with
Held Hein??) and finding things if he drops
them.
26
MT/MST (motion)
V4 (color)
Infero-temporal cortex
27
Implications? Basic object perception
(recognition and segmentation) requires
experience. (Experience prior to 3 yrs not
enough.) Geometric cues about scene structure
(perspective, distance) also require
experience. Color and motion more robust -
either present at birth, or acquired before 3yrs,
and preserved without continued experience.
28

• Lab 3 Learning Visuo-motor coordination.
• Virtual ball catching environment.
• Ball bounces.
• Hit ball with ping-pong bat.
• Possible manipulations
• Move bat in virtual environment so it appears to
  be displaced from actual hand position. Look at
  ability to adapt.
• Change bounciness of ball. How does hand movement
  change?
• Can Ss learn that different balls bounce in
  different places? Red and blue balls with
  different average bounce points. Does fixation go
  to different location for red and blue balls?